This invention relates generally to probe stations for making low voltage, low current measurements on a device under test such as an integrated circuit or a semi-conductor wafer, and more particularly to an improved probe holder for the wafer probe station for using guarding to reduce current leakage during low current measurements and utilizing Kelvin connections to compensate for line resistance voltage drops which would give rise to errors when making low voltage measurements.
The need for, and principles used in, guarded probes with Kelvin connections are well known and laid out in published articles such as "Fixturing for Low-current/low-voltage Parametric Testing", Evaluation Engineering, November 1990, pages 150-153 by William Knauer. The probe holders are generally mounted on probe manipulators and are used in a probe station having a moveable chuck to position the device under test. Triaxial cabling from the measuring instruments is usually led to the vicinity of the probe manipulator, utilizing a pair of triaxial cables for each manipulator and probe holder. The two triaxial cables serve as source and measurement lines, which are necessary to establish Kelvin connections with respect to the probe.
The prior art, illustrated in FIGS. 1a, 1b and 1c has generally terminated the triaxial cable pair in an external box or "Faraday-cage" and then used coaxial cables connected to the respective inner and intermediate conductors of the triaxial cables. The coaxial cables were carried inside a tube running through the enclosure wall to the inside of the probe station enclosure, and run to a probe holder adapted to receive a probe needle.
When making Kelvin connections, it is desirable to make the connection as close to the probed area as possible. In view of the congested area around the device under test, it is undesirable to utilize the standard coaxial receptacles and plugs normally used in making coaxial connections.
Accordingly, one object of the present invention is to provide an improved probe holder for guarding and making Kelvin connections in a wafer probe station.
Another object of the invention is to provide an improved probe holder which reduces congestion in the vicinity of the device under test.
Still another object of the invention is to provide an improved and economical, fully guarded probe holder, with provisions for making Kelvin connections.